Well cementing for oil, gas, or ground water involves mixing a slurry of cement, water, and other additives and pumping it down through a casing to critical points in the oil well annulus around the casing or in the open hole below the casing string. The primary functions of the cementing process are to restrict fluid movement between geological formations and to bond and support the casing. In addition, the cement aids in protecting the casing from corrosion, preventing blowouts by quickly sealing formations, protecting the casing from shock loads in drilling deeper wells, and sealing off lost circulation or thief zones.
A common problem in petroleum well cementing is the loss of filtrate from the cement slurry into porous low pressure zones in the earth formation surrounding the well annulus. This fluid loss is undesirable since it can result in dehydration of the cement slurry, and it causes thick filter cakes of cement solids which can plug the well bore; moreover, the fluid lost can damage sensitive formations. Cement fluid loss is particularly critical in a process known as squeeze cementing.
There is a requirement, therefore, for materials which, when added to the cement formulation, reduce the loss of fluid from the slurry to porous formations. The use of polyvinyl alcohol as a fluid loss control additive for cements used in oil field applications is known. The use of partially hydrolyzed polyvinyl alcohol (84-90%) is particularly common. Polyvinyl alcohol has been used as a fluid loss additive in various forms as is exemplified in the following references.
U.S. Pat. No. 5,728,210, entitled “Composition and Method to Control Cement Slurry Loss and Viscosity” of Moran et al., and U.S. Pat. No. 5,850,880 entitled “Composition and Method to Control Cement Slurry Loss and Viscosity” of Moran et al., utilize polyvinyl alcohol as a liquid fluid loss additive for use in cementing well bores, particularly oil and gas well bores. The liquid fluid loss additive is formed from dissolving partially hydrolyzed polyvinyl alcohol in water. The polyvinyl alcohol has a molecular weight of about 200,000. The polyvinyl alcohol is used in conjunction with a dispersing sulfonated polymer and surfactant. This combination is then adjusted to individual well conditions with chelating agents, cross-linking agents, biocides, antifoams, or combinations of these.
U.S. Pat. No. 6,180,689, entitled “Fluid Loss Control Agents and Compositions for Cementing Oil Wells Comprising said Fluid Loss Control Agent” of Moulin, describes a fluid loss control agent for a petroleum industry (or analogous) cement slurry, comprising a surfactant and a micro-gel obtained by chemical cross-linking of a polyvinyl alcohol. The micro-gel and the surfactant are compatible with petroleum industry cement additives and can also produce compositions which are gas tight. The micro-gel is obtained by reacting the polyvinyl alcohol in solution with agents which can condense with at least two alcohol functions at a pH of less than 10. The micro-gel is typically prepared in aqueous solution comprising 2% to 5% (by weight), preferably on the order of 3.5%, of cross-linked polyvinyl alcohol.
An aqueous gel that is formed from a polyvinyl alcohol or vinyl alcohol copolymer and a partially methylated melamine-formaldehyde resin in the presence of a pH regulating agent is described in U.S. Pat. No. 5,061,387, entitled “Aqueous Gel System of Partially Methylated Melamine-Formaldehyde Resin and Polyvinyl Alcohol” of Victorius. These gel-forming compositions control the permeability of underground formations during water flooding and chemical flooding operations. These plugging techniques are also used during well workovers, for example, to plug leaks in well casings or to temporarily plug wells, in fracture treatments, to consolidate unconsolidated formations, and to correct the injection profile of a well by sealing high-permeability streaks so that flooding fluids will enter the formation in a more desirable front.
U.S. Pat. No. 5,009,269, entitled “Well Cement Fluid Loss Additive and Method” of Moran et al., relates to cementing of a casing string in a well bore, and more particularly to a fluid loss additive for addition to a cement slurry to be used in the cementing job. Moran et al. describes a fluid loss additive which is stated to be effective at temperatures of up to about 200° F., has limited effect on slurry viscosity, and does not significantly retard cement setting. The additive is comprised of a partially hydrolyzed vinyl acetate polymer, calcium sulfate, a cross-linker for the polymer and, optionally, a defoamer. Because of difficulties in manufacturing a PVOH with a molecular weight above about 200,000, the use of PVOH was considered limited to formation temperatures of about 120° F. This disclosure teaches that the useful temperature can be increased to about 200° F. by including cross-linking materials in the additive. However, at temperatures much above 200° F., the cross-linked PVOH is not thermally stable.
U.S. Pat. No. 4,703,801, entitled “Method of Reducing Fluid Loss in Cement Compositions which may Contain Substantial Salt Concentrations” of Fry et al., discloses a process for reducing fluid loss in cement compositions made with salt water. The compositions are comprised of water, hydraulic cement and a fluid-loss additive comprising a graft polymer having a backbone of lignin, lignite, derivatized cellulose and various synthetic polymers such as polyvinyl alcohol, polyethylene oxide, polypropylene oxide and polyethyleneimine. The grafted pendant groups comprise homopolymers, copolymers and terpolymers of 2-acrylamido-2-methylpropane-sulfonic acid, acrylonitrile, N,N-dimethylacrylamide, acrylic acid, N,N-dialkyl-aminoethylmethacrylate and their salts. The backbone comprises from about 5 to about 95 percent by weight of the graft polymer, and the pendant groups can comprise from about 5 to about 95 percent by weight of the graft polymer.
Additional references of interest follow. U.S. Pat. No. 6,110,270, entitled “Method for Influencing Moisture Content and Migration in Building Materials” of Beckenhauer, teaches an aqueous PVOH solution for typical use as a coating on building materials in order to prevent the migration of moisture through a porous building. The solutions may contain from about 0.01% to about 30% by weight of PVOH which may have a molecular weight ranging from about 5,000 to about 500,000.
U.S. Pat. No. 6,739,806, entitled “Cement Compositions with Improved Fluid Loss Characteristics and Methods of Cementing Subterranean Formations” to Szymanski et al., discloses methods for preventing fluid loss in cement slurries by connecting two polymers via a pH sensitive crosslinking agent, such as a polyvalent cation. In preferred embodiments, the additive contains a first PVOH polymer with a molecular weight of at least 80,000 and a second PVOH polymer with a molecular weight of about 8,000. The polymers are dissolved in water with a cross-linker and the pH is adjusted until the solution achieves a desired molecular weight. Likewise, U.S. Pat. No. 5,594,050 to Audebert et al. discloses a fluid loss control agent which employs chemically cross-linked PVOH.
U.S. Pat. No. 5,105,885, entitled “Well Cementing Method Using a Dispersant and Fluid Loss Intensifier” of Bray et al., discloses a fluid loss additive package containing an ethoxylate, a dispersant material and, optionally, a water soluble polymeric compound. The polymeric compound may comprise polymers such as polyvinyl alcohol or 2-acrylamido-2-methylpropyl sulfonic acid (AMPS) copolymers.
U.S. Pat. No. 4,569,395 to Carpenter entitled “Matrix Control Cementing Slurry,” describes the use of a fully hydrolyzed polyvinyl alcohol resin to ameliorate problems with slurry thinning at elevated temperatures. The compositions in the '395 Carpenter references also include, in some embodiments, water soluble cellulosic polymers and dispersants. U.S. Pat. No. 4,967,839, entitled “Method and Composition for Cementing in a Wellbore” of Carpenter et al., discloses a cement composition for oil and gas wells comprising at least 2 weight percent of tricalcium aluminate, at least 2 weight percent of gypsum, and between 0.3 to 2.0 weight percent of a polyvinyl alcohol having a degree of hydrolysis that is less than about 92 percent. According to Carpenter et al., polyvinyl alcohols with a molecular weight of less than 75,000 are preferred.
Despite the above contributions in the art, the use of polyvinyl alcohol as a fluid loss additive suffers from various drawbacks. For example, while partially hydrolyzed polyvinyl alcohol exhibits useful fluid loss properties at lower temperatures (up to about 150° F.), it must be added in progressively larger quantities for moderate or high temperature wells where temperatures of up to 250° F. may be encountered. This is problematic, as excessive amounts of fluid loss additive can negatively affect the rheology of the cement slurry making it difficult, if not impossible, to pump.
Some approaches to improve rheology in cementing processes are described in U.S. Pat. No. 4,997,487 to Vinson et al.; U.S. Pat. No. 5,184,680 to Totten et al.; and U.S. Pat. No. 5,273,582 to Totten et al. These references advocate the use of a viscosifying agent, such as guar gum, with a set retarding composition, such as an AMPS-type resin to improve slurry viscosity properties. U.S. Pat. No. 6,708,760 to Chatterji et al. uses a cellulose derived composition to accomplish similar goals. These approaches also have disadvantages, however, such as retarding the set time of the cement, which is not always desired.
There is accordingly provided in one aspect of the present invention a polymeric fluid loss additive having superior performance over a broad range of temperatures, even when present in modest quantities. The additive has effective fluid loss properties at high temperatures such as 195° F., and preferably up to 250° F. or more. Additionally, cement slurries prepared with the inventive additive generally maintain rheologies that are suitable for oil field cementing operations; thus, the slurries do not exhibit excessive thickening at surface conditions or thinning at higher bottom hole temperatures.